Frank Duignan wrote:
> Check the memory limit for the chipset. I've got a laptop with the Intel
> 945 chipset and it seems that the northbridge is limited to a 32 bit address
> bus so when I put 4GB of RAM in, it could only address 3.3GB as the
> northbridge reserved part of the 32bit address space for PCI stuff, graphics
> etc (no remapping possible).
> So to re-iterate: check the memory addressing capabilities - just because it
> says its 64 bit does not mean it can address more than 4GB
> f.
>> 2009/3/10 Frank Murphy (Frankly3D) <frankly3d at fedoraproject.org>
>bits of CPU is not related to Memory Size ...
A 16 bit CPU with 16Bit addressing can only address 64K (as indeed is
case with 8088 / 8086 really).
Decent 16 bit CPUs can address 16Mbyte.
Some 8 bit CPUs use 808x segment approach and MMU to access 1Mbyte, just
like 8088.
A 32bit address is indeed limited to 4Gbyte. But if each address was a
16 bit word, then 8Gbyte, or if each address a 32bit word, then 16GByte.
Indeed you could have a 128bit CPU with only 16M memory space.
*Physical Address Extension.* PAE is an Intel-provided memory address
extension that enables support of up to 64 GB of physical memory for
applications running on most 32-bit (IA-32) Intel Pentium Pro and later
platforms.
The Pentium Pro is a LONG time ago. Before the inferior PII.
A P4 though while able to have 64GByte of physical RAM, can only have 4G
virtual Memory for each program/Process
Oddly NT4.0 enterprise supported PAE, though the ordinary versions
limited to 4G.
XP and Vista do not according to some. MS claims the 32bit versions
support PAE and 64bit versions don't.
You are limited to about 3.5Gbyte due to memory mapped I/O (a bit like
the 640k vs 1M space of original PC) on most motherboards. I've never
needed more than 512M on Linux in recent times (32M in 1999), nor on XP.
PAE allows the most recent IA-32 processors to expand the number of bits
that can be used to address physical memory from 32 bits to 36 bits.
*READ the motherboard and chipset spec carefully* They may limit a
64bit CPU with 64bit memory bus to 3.5G addressable RAM.
I had a real computer with only 1Kbyte RAM once :-)
--
Mike
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